Cold seal adhesives comprise a special class of pressure-sensitive adhesives which possess the capability of forming a strong bond to themselves when pressure is applied, yet are also capable of being applied to a substrate and reeled (as a dried film) for storage without effecting such a bond. Thus, the adhesive must be sufficiently plastic to form a bond under the application of pressure alone, yet sufficiently hard to resist bonding to a substrate during storage. Such adhesives are used in a variety of applications including as envelope sealants and in food packaging where the application of heat to effect a bond is undesirable (e.g. in candy packagings).
To date, the conventional approach to formulating such adhesives has been to combine a natural rubber elastomer (particuarly a latex) with a tackifier and other compounds. A typical formulation comprises 55-60% (by wt.) of a high ammonia content natural rubber latex emulsion, 30-40% (by wt.) of a styrene acrylate emulsion, and small amounts of wetting agents, latex stabilizers, antioxidants, biocide, and thickeners. Such compounds exhibit good cold sealing properties. However, they also exhibit the poor aging properties of natural rubber based compounds, including softening and discoloration of the adhesives, possess an objectionable odor, and exhibit undesirable foaming when in the liquid state.
Attempts to replace natural rubber with synthetic rubbers to alleviate these problems has proven to be difficult, however, due to the poor strength of the resultant bonds. Thus, there exists a real need for synthetic-based cold seal adhesives which exhibit good bond strengths.